1. Field of the Invention
The present invention relates generally to an implantable heart defibrillator of the type including means for sensing the occurrence of fibrillation and for emitting at least one defibrillation shock in response, and to a method for defibrillating a heart.
2. Description of the Prior Art
A modern automatic, implantable defibrillator, e.g. as described in Current Problems in Cardiology, Volume XIV, No. 12, Dec. 1989, Chicago, Troup J. P. "Implantable Cardioverters and Defibrillators", p. 699 ff, especially FIG. 14 and its legend, includes cardioversion and pacemaker stimulation capability for both tachycardia and bradycardia, in addition to a defibrillation function, and is sometime referred to as an AICD (automatic implantable cardioverter defibrillator). In AICD defibrillation, an attempt is made to induce all heart cells to depolarize simultaneously by imposing a strong electrical field across the heart, i.e. the heart is electrically shocked. The electrical shock takes the form of electrical pulses which can sometimes be emitted in patterns of varying spatial and temporal sophistication.
Achieving depolarization of heart cells by mechanically touching them is also known. The result is a heartbeat achieved mechanically. One possible explanation of this phenomenon is that mechanical contact causes leakage from the cellular ion channels. A change then occurs in ion concentrations inside and outside the cell membrane, thereby triggering alepolarization and thus a heartbeat.
This knowledge regarding mechanical depolarization has, however, not been employed in the art of defibrillation which, with the exception of devices for injecting fibrillation-terminating drugs into the heart, (e.g., European Application 0 429 141), has concentrated exclusively on defibrillation by electrical shock. Considerable electrical energy (about 5-40 J) is required for defibrillation. Tissue subjected to such energy in the form of an electrical shock could be damaged. For this reason, efforts are being undertaken in the art in an attempt to reduce the electrical energy needed for defibrillation.
One procedure for achieving this reduction mechanically is described in U.S. Pat. No. 4,925,443, which relates to an artificial, implantable mechanical support function for ventricular compression of the heart. However, any defibrillation necessary in the patient is still performed electrically. The mechanical support function is used in conjunction with de,fibrillation only to compress the heart so a lower level of electrical defibrillation energy can be used.